This is sandwich that my grandfather learnt from an American soldier whilst he was studying in the U.S.
According to my grandmother, a young American soldier that they invited over brought this sandwich as a gift. Although that sandwich contained oiled sardines and 1 cm-thick onion slices.

Add plenty of onion, seasoning and mayo for flavor.
Apparently, the American soldier that befriended my grandfather used thickly cut onions, but you can cut them to whatever size you like. Recipe by Setsubunhijiki